1. Field of the Invention
The invention pertains to a device for the cooling of an X-ray source of the type where an X-ray tube is cooled by means of a fluid put into forced circulation.
2. Description of the Prior Art
An X-ray source comprises an X-ray tube contained in a fitted-up casing. The casing acts as a shield with respect to X-rays, and electrical and mechanical shocks. It is becoming increasingly frequent for the X-ray source to comprise also a system to cool the X-ray tube and the casing. This cooling is necessary because the electrical energy used to produce X-rays is converted into X-radiation with an efficiency of about 1%, i.e. 99% of this energy is converted into heat inside the source.
In X-ray applications entailing a very small work load, i.e. where the rate at which shots are taken is slow and corresponds to a mean power dissipation of about 200 Watts, natural convection phenomena are enough to do the cooling. A small fan near the casing further increases convection around the said casing so that the mean power dissipation reaches about 400 watts.
With modern X-ray diagnosis methods using, for example, vascular examination, X-ray cinematography or scanners, the shots are taken at a very fast rate and may entail a mean power dissipation of several thousands of watts. For X-ray applications of this type, the cooling systems are far more complex and bigger, and the operation and performance of these x-ray diagnosis installations, where the thermal load of the X-ray source is very high, is conditional upon the efficiency of these cooling systems.
The most widespread cooling method used, when the thermal load is high, consists in cooling the X-ray tube by using a fluid already contained in the casing in order to provide electrical insulation. This fluid may be oil for example. The fluid or oil is put into forced circulation around the X-ray tube and, outside the casing, where it flows into a cooling circuit comprising a heat exchanger. The fluid or oil, having received the heat produced by the X-ray tube, is cooled in turn when it flows into the heat exchanger. The heat exchanger may be, for example, of the oil-air exchanger type or again, of the type comprising a second circuit in which there flows a second cooling fluid such as water, for example.
Thus the X-ray tube and the casing are cooled by the oil which flows through the casing, the oil itself being cooled by means of the heat exchanger, the dimensions of which should make it possible to remove the heat produced by the power dissipated during an operating cycle of the X-ray source.
In radiology, and especially in scanner applications, an operating cycle consists of two consecutive periods. The first of these periods corresponds to the intensive operation of the X-ray source and is called the examination period. The second period is called the idle period and corresponds to a stoppage in the operation of the X-ray source. With scanners, the examination of a patient calls for a great many sectional views, one after another, so that during the examination period the heat load is extremely high. Then, between two examinations of patients, i.e. during the idle period, no heating is given to the X-ray source. Because of this, the heat exchangers used in the prior art are over-sized with respect to the power dissipated during an operating cycle. Consequently, these heat exchangers have certain disadvantages, in addition to being to their high cost ad excessive bulk and weight which entail the use of heavy complicated mechanical means to make the casing movable.
With this method the oil, in flowing through the casing, receives a quantity of heat Q during the examination period. This heat Q is divided into two parts Q1 and Q2. The first part Q1 is removed by the heat exchanger during the examination period. The second part Q2 raises the temperature of the casing/cooling circuit unit by a value .theta. such that: EQU .theta.=(Q2/.mu.), (1)
where .mu. is the equivalent thermal capacity of the unit.
It mst be noted that, in the prior art, the method used to avoid having heat exchangers of absolutely unacceptable dimensions is to increase the thermal capacity .mu. of the unit formed by the casing and the cooling circuit, by increasing the volume of fluid or oil used to cool the casing and the X-ray tube. This method, apart from increasing the volume and weight of the said unit, has the disadvantage of reducing the efficiency of the heat exchanger.
For, if we assume, for example, that the heat exchanger is of the oil/outer air type, the quantity of heat that the said heat exchanger can be used to dissipate is proportionate, according to an initial estimate, to the difference in temperature between the fluid or oil flowing through the heat exchanger and the outer air.
Furthermore, the rise in the temperature of the unit from a starting temperature .theta.1 up to a maximum permissible temperature of .theta.m is written: ##EQU1## where Pe is the mean power during an examination, T1 is the period corresponding to the examination, .alpha. is the exchange coefficient of the heat exchanger and .mu. is the thermal capacity of the unit. It follows from the first relationship (1) above that the exchange coefficient .alpha. should be as great as possible to limit the rise in temperature of the unit, the said coefficient .alpha. being limited, firstly, by the size of the heat exchanger and, secondly, by the maximimum permissible temperature of the unit.
It can also be seen from the second relationsup (2) above that the greater the thermal capacity .mu., the slower is the rise in temperature. Thus, in the prior art, the increase brought about in the thermal capacity .mu. in order to limit the dimensions of the heat exchanger is such that the maximum permissible temperature is reached at the end of the examination period T1. As a result of this, it is only at the end of the examination period T1 that the exchange coefficient .alpha. is at its greatest. This leads to increasing the size of the exchanger so that a part of the advantage obtained by increasing the volume of oil is lost.
3. Summary of the Invention
The present invention pertains to a device for the cooling of an X-ray source which can be used to obtain the efficient cooling of the casing and the X-ray tube, using a small volume of fluid or oil to cool the casing and the X-ray tube while, at the same time, using a heat exchanger which is small when compared with heat exchangers used in the prior art. This result is obtained by a new arrangement of means which makes it possible, in particular, to store heat during the examination period and then to restore this heat to the heat exchanger between examination periods, so that the heat exchanger works as efficiently as possible at an almost continuous rate.
The invention pertains to a cooling device for an X-ray source comprising a casing containing an X-ray tube that works with a heat load which is higher during an examination period than during an idle period which follows the said examination period, the casing further containing a fluid to which the X-ray tube yields its heat, the fluid being put under forced circulation along a given direction in the casing and in a cooling circuit comprising a heat exchanger, the heat accumulated by the fluid comprising a first quantity of heat which is partially removed by the heat exchanger and a second quantity of heat which tends to raise the temperature of the casing/cooling circuit unit, wherein the cooling circuit comprises means which firstly store a third quantity of heat accumulated by the fluid when the said fluid reaches a pre-determined temperature and, secondly, to restore this third quantity of heat to the heat exchanger by means of the fluid during the idle period which follows the examination period.